1. Field of the Invention
This invention relates to a personal authentication medium issuing apparatus and personal authentication medium issuing method which record synthesized image information formed by embedding and synthesizing additional sub-information (such as security information) set in an invisible state into main image information (such as a face image of a human being) and issue a personal authentication medium, printed matter, and truth determination apparatus and truth determination method which determine the truth or not of the printed matter.
2. Description of the Related Art
In recent years, greater importance is attached to the technique for making an electronic watermark or electronic signature in order to prevent forgery or falsification of an image as information is more electronized and the Internet is more widely used. Particularly, the electronic watermark technique for embedding additional sub-information (sub-image information) in an invisible state into main image information is proposed to take a countermeasure against illicit copying, forgery or falsification of a personal authentication medium such as an IC card or a photograph of having copyright information embedded therein.
For example, an electronic watermark inserting method is known which embeds data into image data output to a printing matter by use of the characteristic of a color-difference component or high spatial frequency which is difficult to be sensed by a human being. Further, an electronic watermark printing apparatus which can confirm an electronic watermark by use of an optical filter is known.
In the conventional electronic watermark embedding process, sub-information is embedded into main image information in an invisible state by use of the following characteristics (1), (2) and (3).
(1) The visual characteristic of a human being is utilized.
In this case, as the visual characteristic of a human being, it is utilized that the gradation identification ability is lowered with an increase in the frequency of an image and it is more difficult to determine color-difference information than luminance information.
(2) The relation of complementary colors is utilized.
For example, red and cyan (=green+blue) are set in the relation of complementary colors in the case of an additive color mixture. Therefore, when red is adjacent to cyan, it is impossible to individually discriminate the colors to the human eyes and they can be seen as an achromatic color.
(3) The relation of complementary colors and color-difference information are applied to a high frequency carrier pattern image.
For example, when a block (high frequency carrier pattern image) in which red-rich images and cyan-rich images are repeatedly arranged is observed by human eyes, the color-difference amount is determined (visually recognized) as ±0 (that is, as an achromatic color). This is because a small difference in the color difference cannot be recognized by the human eyes.
The truth determination of a printed matter having an image recorded thereon (an image which has sub-information embedded in main image information) to which the above electronic watermark technique is applied is made by checking whether the sub-information obtained by a sub-information restoring process is correct or not. By performing the restoring process by use of key information, the sub-information recorded in an invisible state is restored and the truth determination is made.
However, in the electronic watermark technique utilizing the color difference, the stability (of restoration) of sub-information may vary depending on a color of the main image information in some cases. For example, when a face image is used as the main image, the image quality of a black portion such as hairs is degraded and the stability of the sub-information is degraded in some cases. Also, when sub-information is embedded by use of the same color-difference amount as that of the face portion of a skin color, the stability of the sub-information is degraded in some cases. In a printed matter such as a personal authentication medium, in order to make stable and correct truth determination, it is necessary to set a state in which sub-information embedded in the main image information can be correctly restored. Therefore, conventionally, in the issuing process of a printed matter such as a personal authentication medium, it is necessary to check whether or not the sub-information can be correctly restored from the entire personal authentication medium formed. The above operation takes an extremely long time and may cause the issuing efficiency of the printed matters such as the personal authentication media to be extremely lowered.